In a plain-paper copying (PPC) machine toner images applied to the surface of paper or other recording medium are fixated by application of heat and pressure. In certain PPC machines toner fixation is accomplished by passing the image-bearing recording medium between a hot thermal-fixation roll and a pressure roll to fuse the toner in place so that it is not easily removed from or is difficult to smear on the surface of the paper or other recording medium. When this type of thermal-fixation device is used the toner material is directly contacted by a roll surface and a portion of the toner adheres to the roll surface. Unless this is controlled, with subsequent rotation of the roll the adhered toner material may be redeposited on the recording medium resulting in undesirable offset images, stains, or smears; or, in severe cases, the recording medium may stick to the adhered toner material on the roll and become wrapped around the roll.
To perform satisfactorily in a thermal fixation assembly the material forming the pressing surfaces should be sufficiently temperature resistant to operate at the temperatures required to fuse the toners, capable of complying to and applying uniform pressure to the toner images, and have, or have imparted to their surface, release properties that minimize toner pickup. Additionally, these materials should be sufficiently durable in performing these fucnctions to be cost-effective.
In the past, solid rolls consisting of elastic materials such as urethane rubber, ethylene propylene rubber, silicone rubber, or fluorocarbon rubber have been used. Such elastic materials, when used alone, suffer the drawback of having relatively poor release properties and toner particles, paper particles, and the like, would cling to the rolls and lead to reduced image quality and a shorter use-life. When used in conjunction with release agents, for example, silicone oils applied to their surfaces, or release agents present in oil-less toners, improved release properties were obtained, however, often at the expense of durability as many of the elastic materials were degraded by the release agents and failed prematurely.
To overcome these problems solid rolls of elastic materials with a covering layer of fluoropolymer film or shrink-fit tubing have come into use. Fluoropolymer materials such as tetrafluoroethylene/hexafluoropropylene copolymer (FEP), tetrafluoroethylene/(perfluoroalkyl) vinyl ether copolymer (PFA), and polytetrafluoroethylene (PTFE) are well known for their temperature resistance, chemical stability, and excellent release properties; and their use in solid rolls have resulted in improved release properties and heat resistance, although at considerable sacrifice of mechanical properties such as elasticity, compliance, and surface hardness.
New needs in the printing and copying industry, for example, the desire for higher printing and copying speeds, more compact and lighter equipment, and the desire to print or copy onto surfaces of non-uniform thickness such as are created by envelope flaps, and the like, are such that even the improved roll materials no longer perform entirely satisfactorily. Higher operating speed may require higher operating temperature in order to accomplish image fixation with a shorter residence time at temperature and pressure. Higher operating temperatures may lead to reduced use-life in the materials. Altematvely, to operate at high speeds but at lower temperature, the residence time at temperature and pressure can be increased by increasing the nip width between the pressing surfaces. Generally, this can be done by increasing the contact pressure, which can lead to premature failure of the material; or by increasing the diameter of the roll, which conflicts with the desire for compactness.
To address these needs rolls made of elastic materials in the form of flexible porous foams have been developed. Rolls made of elastic porous foam materials have elasticity and compliance properties which allow the nip width to be increased without increasing the contact pressure between the pressing surfaces or increasing the roll diameter, and can print or copy onto substrate surfaces of non-uniform thickness without creating wrinkles in the image-bearing substrate. However, when made of the elastomeric materials described above, they suffer the same drawbacks of poor release properties or degradation associated with release agents. When covered with a layer of fluoropolymer material as described above the elasticity and compliance of the elastic foam body are compromised as the higher hardness and rigidity of the covering layer prevents the elasticity and compliance properties of the elastomer foam from being adequately exploited. Furthermore, because of the disparity in the physical properties of the materials, substantial stress is developed at the interface between the covering layer and the foam body so that the covering layer tends to separate from the foam body, and failure occurs.
It is a purpose of the present invention to provide an elastic fixing roll which has excellent release properties, surface contact characteristics, heat resistance, and durability; and which can satisfactorily address the current needs described above.